The simplicity and availability of the standard roller pump have made it the pump of choice for extracorporeal circulation. This pump is widely used in dialysis, routine cardiopulmonary bypass and long term pumping such as extracorporeal membrane oxygenators, (ECMO) and left and/or right heart bypass. The standard roller pump maintains a constant flow independent of clinically expected changes in inlet or outlet pressures. Thus, a decrease in blood supply at the pump inlet, without a concomitant decrease in pump speed, can cause excessive suction leading to air embolism, thrombosis and damage by the "venous" cannula to the vessel's intima. The combination of constant flow and an arterial line that is accidentally clamped or kinked, or an arterial cannula that is positioned against the vessel intima, can generate excessive pressures at the outlet of the pump which at the extreme, can blow up a connector, tube or an oxygenator.
To overcome these potential dangers in closed systems such as ECMO or dialysis, collapsible bladders have been placed at the inlet to the pump such that at too high a suction, the bladder collapses actuating a microswitch which stops the pump. The pump restarts when the bladder refills. Others have designed roller pumps with a servomotor and a microprocessor control system. When the roller pump is used during cardiac surgery for venting the left ventricle or for returning shed blood from the chest cavity it requires constant surveillance by a trained perfusionist to assure that no excess suction is applied.
Presently protection against extreme inlet or outlet pressures that may be generated by the roller pump require either on-off control with the standard roller pump, extensive and expensive modifications to the standard pump (e.g., CAPS Stockert-Sorin, Sorin Inc. Irvine, Calif. 92714) or a watchful perfusionist. These techniques require invasive pressure measurement which has the disadvantage of requiring a sterile transducer and promoting stagnating blood in the pressure monitoring line that can lead to thrombosis. A heparin coating that inhibits thrombosis to the inner walls of the extracorporeal circuit cannot be applied to the open port used to measure pressure. U.S. Pat. Nos. 4,515,589 and 4,767,289 (manufactured by Sarns/3M Corp. as the "Safety Loop"), and 4,650,471, describe devices to be used with the roller pump that prevent too much suction. The former provides no adjustment over the pressure about which flow is controlled. The '471 patent describes adjustment capabilities for the inlet, but neither provides relief for overpressurization at the outlet of the pump. Another solution is to use a centrifugal pump such as that made by Biomedicus of Minneapolis Minn. Its flow characteristics permit a limited negative pressure and outlet pressure to be generated, its costs however are high, about $10,000 for the pump module and $200 for each of the disposable pump heads. It also can generate a negative pressure of 600 mmHg and an outlet pressure of over 800 mmHg, both of which valves exceed clinical usefulness and can be traumatic.
Hemolysis by roller pumps is due to crushing of blood cells between the walls of the tubing being squeezed and/or high shear rates possible with retrograde flow through nonocclusive tubing. Pump occlusion is set by measuring the drop rate of a column of liquid at the outlet of a stopped pump. Drop rates anywhere from 1 to 40 cm/min per 100 cm pressure difference are reported in the literature. The drop method has five major disadvantages: inaccuracy because of relatively large variation in tubing wall thickness (+/-0.003"), unequal extension of the two rollers, off center roller rotation, pump raceways that are not truly circular and, during fast drop rates, the pressure decreases as the liquid falls. Thus, proper occlusion setting requires averaging of multiple readings which is a time consuming effort. Although it is reported by Bernstein and Gleason (Factors influencing hemolysis with roller pumps, Surgery, 61:432-442, 1967) and Noon et.al. (Reduction of blood trauma in roller pumps for long-term perfusion, World J. Surgery 9:65-71, 1985) that hemolysis increases as the occlusion is increased, for very nonocclusive settings the drop rate is too fast to measure accurately. Others have suggested that roller occlusion be set by measuring a pressure drop at the outlet of the pump. With this method, the pump outlet is clamped, the pump is rotated to increase the pressure to the desired level, the pump is then stopped, and the rate of pressure drop measured. Occlusion setting by this method is very dependent on the compliance between the pressure transducer and the roller occluding the tubing: the larger the compliance the lower the occlusion. Both aforementioned methods use a stationary pump, referred to hereafter s static tests which rely on a measurement taken from a single point along the pump raceway and from a single roller to determine occlusion. It may be that one of the reasons that users generally do not set the pumps in a less occlusive manner, is the difficulty in doing so accurately. It would be of great clinical advantage to be able to provide control over the maximum pressure in an extracorporeal circuit and the maximum suction the patient is exposed to, as well as to provide a simple means to enable the user to set the pump nonocclusively and provide a standard roller pump with the advantages of a centrifugal pump without its associated high costs. These can be done with pressure sensitive valves and appropriate control devices.
In the medical field, valves known as Starling resistors, are made of a thin walled sleeve and require negligible transwall pressure difference to close them. They have been suggested for use to maintain or adjust pressure, (Robert Rushmore: Control of Cardiac Output, in Physiology and Biophysics 19th edition Ruch TC and Patton HD editors, WB Saunders Co. Phil. 1965).
These valves made of a sleeve sealed in a housing with means to pressurize the interluminal space (the space between the housing and the sleeve). Pressure applied to the interluminal space acts upon the wall of the sleeve forcing the opposite walls of the sleeve to meet and close shut. This external force on the wall is counteracted by the pressure within the lumen of the sleeve which tends to keep the walls apart. It is the net force of these two vectors that determines whether the sleeve is opened, closed or in between.
In industry, these valves are used as ON/OFF valves or as adjustable resistors known as pinch valves. Pinch valves are also used to adjust the resistance to flow using an external roller that pinches and thus controlling the degree of closure of the sleeve. If the wall of the sleeve is made sufficiently thin, the valve can also be used to transfer the pressure of the fluid within the sleeve to the interluminal space without significant changes in the transduced pressure. Thus, these devices can transmit the pressure of a fluid that may be corrosive to a pressure gauge while isolating the pressure gauge from the fluid.
U.S. Pat. No. 4,767,289 teaches that a Starling valve may be made of a thin wall tubing, both ends of which are sealed to a rigid connector which in turn are sealed to the housing providing flow through chamber. U.S. Pat. No. 4,515,589 teaches that the walls of the thin wall tubing may extend beyond the housing, be folded upon themselves and sealed over the external wall of the housing. Another manufacturing technique suggests that the inner wall of a resilient sleeve be affixed to the outer wall of the thin walled tubing and the outer wall of that sleeve be affixed to the housing. These techniques have one or more of the following disadvantages: 1) the thin wall tube is stressed over the edges of the housing, 2) the assembly requires sealing the thin wall tube to the connectors, 3) the discontinuities of the valve at the connection sight between the thin wall and the thick wall tubing can create turbulence and trapped vortices, a leading cause of thrombus generation, 4) the assembly is labor intensive and require multiple parts and 5) control over the interluminal pressure with present systems is provided by a cumbersome and bulky combination requiring a compliance chamber, a pressure manometer and interconnecting tubing.
These disadvantages may be the reasons for the lack of pressure sensitive valve available for clinical use. U.S. Pat. No. 4,250,872 by Tamari illustrates a valve made of unitary tubing a portion of which has been expanded and thinned walled to allow easy contraction by external fluid pressure. This valve however was not made to fully close (as illustrated in FIG. 5) nor was it preformed to close completely shut. The only pressure sensitive valve that is known to be used clinically in the extracorporeal circuit is the one incorporated at the inlet to the "Safety Loop" mentioned above. Its assembly is labor intensive and requires multiple parts. In addition, its housing is exposed to atmosphere and provides no mechanism to adjust the interluminal pressure. Senko Medical Instrument Mfg. Co., LTD. of Tokyo Japan manufactures a pressure relief valve intended for dialysis. It is made by interposing a thin wall a diaphragm made of a plastic sheet between the wall with the pressure port and the blood path. This method though very adaptable to mass production, results in a diaphragm that often is accidentally heat-sealed to the housing wall, preventing its free motion, thereby rendering it nonfunctional. It also has discontinuities at the connection sight between the thin wall and the thick wall tubing creating the areas of stagnation which are prone to thrombus formation.
The present invention discloses the use of elastic sleeves to set the pressure characteristics of a Pressure Relief Valve. Inflatable elastomeric bladders have been used for pressure indication and regulation (e.g., part no. BSVD-300, Shiley, Laboratories, Irvine, Calif., and U.S. Pat. No. 3,993,069). The pressure-volume characteristics of these balloons have a high or low frequency hysteresis, and/or the highest pressure occurs upon initiation of inflation and thereafter it decreases (e.g., see FIG. 1 of U.S. Pat. No. 3,993,069 and FIG. 6 of Tamari's U.S. Pat. No. 5,013,303). The Shiley device is a spherical balloon and therefore upon inflation its size, but not its shape, changes. Neither of these devices allow the user to adjust the pressure. It would be useful to have a pressure regulator with a shape change which indicates pressurization more clearly. It would also be of great advantage to provide the user with the means to adjust the maximum outlet pressure of pumps and incorporate inexpensive means to measure noninvasively the pressure of the pumped fluid.